Process for treating metal surfaces



Patented Sept. 10, 1940 UNITED STATES PROCESS FOR TREATING METALSURFACES Ralph K. Clifford, Kokomo, Ind., assignor to Continental SteelCorporation, Kokomo, ImL, a corporation of Indiana No Drawing.Application August 26, 1935, Serial No. 37,937. Renewed November 20,1939 6 Claims.

This invention relates to a process for treating metal surfaces so thata coating applied thereto will adhere firmly to said surface, and thepresent application is a continuation-in-part of my it earlierapplication filed April 5, 1933, under Serial No. 664,668.

The invention has for one of its objects the preparation of the surfaceof iron or steel sheets to provide for film adherence of zinc coatings,

especially coatings of the heavier types, such, for

example, as are used for the manufacture of culverts.

A further object of the invention is to prepare steel surfaces of thehighly finished type, such as w are produced in strips, both narrow andwide, and on highly finished sheets produced on mills of either thecontinuous, semi-continuous or intermittent types, so that aftertreating said surfaces, zinc coatings will adhere firmly when sub- Wjected to fabrication.

Another object of the invention is to eliminate the use of strong acidsand/or prolonged immersion for pickling purposes.

For the purposes of illustration only, I will describe my process inconnection with zinc-coatin applied to iron or steel, sheets, althoughit will be obvious that this process may be used in connection withother metallic products or for coatings other than zinc, whether oi ametallic nature such vas lead, or of the paint, enamel, or oxide types.Furtlrierrnore, this process may be used to prepare metallic surfacesfor electroplating as well as for hot or molten coatings.

The ordinary practice used for galvanizing (zinc-coating) sheetsconsists in substantially the following procedure:

1. Removal of scale and etching of the sheet surface by means of acid.This is called pickling. (If the sheet has been oiled or greased, a

de-oiling process may be used before pickling.)

2. Removal of acid from the surface by rinsing in water.

8. Neutralizing the remaining surface acid by rinsing in a neutralizingsolution such as a sodium carbonate solution.

4. Soaking in water to eliminate further acid and hydrogen-or 4 Limingand baking-or 50 4 Liming and baking followed by washing and drying-or4. Washing and drying and eliminating step 4.

5. Immersion in a very weak acid solution, 55 on a fluxing solution,prior to 6. Immersion through a molten flux into a bath of molten zinc.

'7. Removal from the molten zinc, followed by- 8. Cooling, or, somespecial treatment and 5 then cooling.

The ordinary process possesses a number of disadvantages due to severalcauses, only a few of which will be mentioned here.

In the first place, the sheets to be pickled are m placed in crates, thesheets standing on edge in said crates. The crates and sheets are thenimmersed in the acid solution, usually sulphuric acid, the strength ofwhich varies but which may be about six (6) per cent of acid by weight.alloy steels a stronger sulphuric acid solution (about per cent) may beused, followed by nitric acid wash. Sometimes a mixture of acids isused.

The crates are moved mechanically to allow at the acid to move betweenthe separate sheets or else the sheets remain stationary and the acid iscaused to move therebetween.

In either event it is diificult to keep the sheets apart during thewhole of the pickling opera- 2d tion and consequently. the sheets areunequally pickled. If the acid is too strong or if the sheets areallowed to remain therein for too long a time the sheets become acidburned. Then, too, the black coating formed by pickling is not allthoroughly removed in the subsequent after treatments.

When continuous processes are used, then the crates are not necessary,other means being then employed to support the sheets, strip or Wirepassing thru the pickling solution. This process also has a number ofdisadvantages especially for material which is to be coated.

Furthermore, steels'o-f different types of manufacture and of differentanalysis do not pickle no as easily nor as uniformly as some others andthe results show up later in poor adherence of the galvanized coatingwhen subjected to fabri-' cation.

When it becomes necessary to subject sheets to a severe pickle inorder'to clean their surfaces then the hydrogen absorption is increasedwith a consequential increase in blisters brought out by the subsequentcoating process.

Certain types of highly finished surfaces, such for example, as thoseproduced in strip mills, or continuous sheet mills, do not lendthemselves to galvanizing by the usual processes used and, so far as Iam aware, until the development of my process, no heavy type coatingscould be For m applied to these highly finished surfaces which wouldwithstand subsequent fabrication, without flaking and cracking off ofthe zinc coating.

The use of my process obviates all of these disadvantages.

In carrying out my process, I may first subject a metal sheet to a lightpickling to remove the major portion of the scale and thereafter, as thenext step, I treat one or both surfaces of the sheet, from edge to edge,-to a pelting process that I will call mechanical etching, to produce asubstantially uniform matte surface which is characterized by amultitude of alternate points and valleys or pits, the points being ofabout even elevation and spacing. This mechanical etching is preferablycarried out through the use of a suitable granular cutting agent ofpredetermined size, directed with force against the surface of the sheetas by a blast of air. The cutting material or etchants, while in a loosecondition, may be applied otherwise to the surface of the sheet, as, forexample, by the use of some holding material, or by centrifugal force.The etchants may be sand, metallic particles of a hard nature,refractory materials, or the like, depending upon the material andcharacter of the surface to be treated, so as to produce a mechanicaletching of the kind above defined. It is also important that the mesh ofthe etchants be suitably chosen for the work in hand, in order toproduce uniform results. For example, a large or coarse mesh etchantwill produce a matte surface which is deeper and rougher than will asmall or fine mesh etchant.

The amount or extent of such mechanical etching by pelting, to producethe required height and depth of the adjacent points and valleys whichcharacterizes my uniform matte surface, depends somewhat on thecharacter of the surface to be treated, as well as the weight of coatingwhich is to be placed thereon. It is desirable, for example, that agreater amount of mechanical etching should be applied for a coatingwhich is heavier in weight. I am not at present aware of the exact rationecessary between the amount of treatment to produce the required relieftexture and the weights of coating to be applied, but I do know thatsome such relationship exists. If the surface of the sheet to be coatedis such that little or no scale is present, or if a heavy coating isdesired, I may dispense altogether with a pickling operation, and employonly the mechanical etching treatment, as the scale need not becompletely, but

should be substantially, removed, to permit of the production of auniform relief texture.

Following a mechanical etching as above described, it is important thatthe sheet be either Washed and dried; that its treated surface receive ablast of air, steam, or some other gaseous medium; that a light picklingsolution be applied; that-the sheet be subjected to a fluxing solution;or that combinations of two or more of these treatments be employed tothoroughly remove any and all fine particles of etching material orfinely divided steel or scale which may remain as a powdery substancewithin the multitude of valleys, or as a coating on the multitude ofhigh points, which characterize the relief matte surface of the sheet.This step of removing the powdery substance from the mechanically etchedsurface is important in order to realize fully the advantages of thisinvention, otherwise the coating may not firmly adhere to the base inthe manner described. By following,

the directions herein given, it will be found that the coating willbecome firmly bonded to the mechanically etched surface, and will remainadhered thereto even under conditions of bending or fabrication.

This process may be carried out as a step by step process, or it may becarried out continuously. In the latter event, the sheet passes firstthrough the pickling solution (if necessary), then through a washing anddrying apparatus, then through a mechanical etching apparatus, thenthrough a wash tank, from which it passes into the molten galvanizingbath. A fluxing solution may be used following the wash tank and beforeentering the molten fiux on the ordinarymolten galvanizing bath.

It will be obvious to those skilled in the art that one or more of theabove steps, either prior to, or subsequent to, the mechanical etchingmay be obviated depending on the condition of the material being coatedand the kind of coating desired.

I have found that by the use of my process I am able to attach heavymalleable zinc coatings to sheets which-will withstand fabricatingoperations heretofore considered impossible.

This process may be used also for continuous wire galvanizing, in whichcase I may eliminate acid pickling entirely prior to coating, in amanner similar to that described for sheets.

The process ascarried out by my invention contemplates mechanicaletching, per se, as a tool for use in preparing surfaces as a part of myprocess for forming adherent, malleable coatings, particularly those ofzinc, or lead, on metallic bodies where heretofore such has not beenaccomplished either practically, economically or commercially.

I claim:

1. In the manufacture of coated sheet metal, the steps includingpickling base metal in sheet form, centrifugally blasting one side onlyof the pickled base metal, and coating both sides of the base with aprotective metal.

2. As a new manufacture, coated sheet metal comprising a ferrous basehaving one side pickled and the other side blasted, and a protectivemetal coating covering both sides.

3. In the manufacture of sheet metal, the process which consists inrolling a ferrous base metal, abrading the rolled surface with acentrifugal blast to provide a matte surface free from hydrogeninclusions and moisture, and then passing the sheet metal through amolten bath of coating metal.

4. The method defined by claim 3 wherein only one side of the base issubjected to abrasion prior to coating.

5. The method defined by claim 3 wherein the base metal is subjected toabrasion with the scale or oxide resulting from rolling, remainingthereon.

6. The method defined by claim 3 wherein the base metal is pickled andthen subjected to abrasion on one side only before coating.

RALPH K. CLIFFORD.

